One unconscious person was found in the sea on Wednesday and later declared dead after the aircraft crashed off the island of Yakushima, according to the Japanese coastguard.

The US Air Force Special Operations Command put the number on board the CV-22B Osprey at eight, saying the craft from Yokota Air Base in Japan "was performing a routine training mission".

The Japanese coastguard referenced only six airmen on board in a statement after having earlier cited eight.

"The cause of the mishap is currently unknown," the US command said in a statement Wednesday, with emergency personnel "on scene conducting search and rescue operations."

An emergency management official in the Kagoshima region where the crash took place said police had received information that the aircraft had been "spewing fire from a left engine".

Photos released by the coastguard showed what appeared to be an overturned yellow life raft and other debris in the water off Yakushima, which lies south of Japan's southernmost main island of Kyushu.

A coastguard spokesman told AFP on Thursday that the search operation had been continuing through the night and that "aircraft and ships are being deployed".

- String of crashes -

The Osprey, developed by Bell Helicopters and Boeing and which can operate like a helicopter or a fixed-wing plane, has suffered a string of fatal crashes over the years.

In August, a crash in northern Australia killed three US marines among the 23 on board.

The Boeing MV-22B Osprey crashed on Melville Island, north of Darwin during a military exercise for locally based troops. At the time the cause was unclear.

Four US Marines were killed in Norway last year when their MV-22B Osprey aircraft went down during NATO training exercises.

Three Marines were killed in 2017 when an Osprey crashed after clipping the back of a transport ship while trying to land at sea off Australia's north coast.

And 19 Marines died in 2000 when their Osprey crashed during drills in Arizona.

- Grounded -

In 2016, an MV-22 Osprey crash-landed off Okinawa, prompting the US Marines to temporarily ground the aircraft in Japan after the accident sparked anger among locals.

Prime Minister Fumio Kishida suggested on Wednesday that his government would not immediately ask the US military to ground its Ospreys or consider doing so for Ospreys used by the Japanese Ground Self-Defense Force, the Kyodo news service reported.

"It is an issue we should think about after confirming what has actually happened," the premier told reporters, according to Kyodo.

The US military has around 54,000 personnel in Japan.